Three days before world leaders formally unveiled an interim deal in early 2014 to slow the growth of Iran’s nuclear program, two House Democrats quietly met to start discussing how they could sell a final accord — if it ever came to fruition — to a skeptical Congress.

It was Jan. 9, 2014, and Texas Rep. Lloyd Doggett and North Carolina Rep. David Price knew that without a sustained campaign from allies in Congress, it would be tough for President Barack Obama to persuade lawmakers to support the agreement over the strong objections of pro-Israel groups like the American Israel Public Affairs Committee.

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“We knew we really needed to get a message going that was strongly in favor of diplomacy,” Doggett said. “And now we are in the position to go back to folks and ask what areas they have questions about. The only alternative is war.”

The deal faces a make-or-break vote this September, when Republicans will move forward with legislation to scuttle the agreement. The GOP is trying to build a veto-proof majority, and both sides, along with Obama himself, are lobbying hard for every vote, with Doggett, Price and about a half-dozen other Democrats quarterbacking the effort to keep Obama’s legacy-defining diplomatic achievement afloat in the House.

Since that early meeting, their campaign has developed into a more organized and formal whip operation. Instead of meeting in Price or Doggett’s offices or on the House floor, the campaign to sway House Democrats to support the controversial Iran nuclear deal is now housed in the Capitol offices of Minority Leader Nancy Pelosi.

Price and Doggett have marshaled a broad array of support in the caucus, from moderates like themselves to liberals in the whip team — all the way up to Pelosi herself, who announced her strong support for the accord shortly after it was announced on July 14.

“A nuclear Iran is unacceptable to the United States, to the world and, in particular, to Israel,” the California Democrat said then.

Pelosi joins an interesting collection of advocates. Doggett and Price are far more moderate than the whip operation’s other leaders. Other deal backers, such as Reps. Barbara Lee of California, Jan Schakowsky of Illinois, Peter Welch of Vermont and Keith Ellison of Minnesota, are some of Congress’ most liberal members.

Buoyed by personal lobbying from Obama, members and Democratic staff say they already have the votes necessary to sustain a veto from Obama if Republicans, as expected, advance legislation to disapprove of the agreement. Obama would need close to 145 Democrats to back the deal if that happens, and senior aides have pointed to the 152 Democrats who signed an early 2015 Price-Doggett-Schakowsky letter supporting the framework of the agreement as evidence they can back the president.

But with anti-deal groups like AIPAC pouring $20 million to $40 million into TV advertisements blasting the deal, Democratic lawmakers said in interviews they weren’t taking any chances. Shortly after Obama announced during a 7 a.m. White House news conference that negotiators in Vienna had sealed a deal, the half-dozen lead supporters in Congress touched base about re-engaging Democrats who had signed onto the letter sent earlier this year.

“We’re having regular meetings and trying to find out what [lawmakers’] concerns are. Most of the members right now are going through the due diligence phase, so part of the whip operation is basically finding out what’s on their mind and how can we get the information to you that you need to come to a conclusion,” Welch said. “There has been a coordinated approach with the White House, and it’s created a real infrastructure of common effort as we come down to the crunch.”

He said Obama’s chief of staff, Denis McDonough, is the group’s main point of contact with the White House.

Over the August recess, there is talk of holding conference calls with members who are still weighing whether to support the deal. Those calls would connect deal experts or the lead negotiators of the accord — Secretary of State John Kerry and Energy Secretary Ernest Moniz — with any members who still have outstanding questions.

The whips are also working with pro-deal groups like J Street — a liberal pro-Israel organization — to help counter the money and energy AIPAC and anti-agreement groups are aiming at swing districts. J Street is planning to spend far less money — just over $1 million — than AIPAC, but Schakowksy said grass-roots support could counterbalance the cash disparity.

Since the deal was announced, Pelosi has taken the reins on outreach to members — an effort the other pro-deal leaders described as critically important to bringing members on board. Meanwhile, Kerry gave a closed-door presentation to House Democrats on Thursday, and more briefings are expected from Obama administration officials before lawmakers leave on their five-week August recess on Thursday.

State Department official Wendy Sherman — who led negotiations with Tehran for the U.S. — will brief the Democratic and Republican leadership teams on Wednesday. That classified meeting will include the committee chairs and ranking members of the House’s national security committees.

The whips are also working members over with a blunt political message: Come out in support now and save yourself from a painful August.

“We’re getting a lot of positive responses where it’s clear that people want to be supportive, especially as they focus on the question of what’s the alternative,” Welch said. “People are taking it quite seriously. This is an extremely momentous decision that the Congress is going to make.”

Any on-the-fence Democrat should expect AIPAC and its coalition of pro-Israel groups to use the August break to their advantage. Beyond the television and Internet ads, the group will capitalize on its national network of allies to apply consistent pressure on lawmakers to vote against the nuclear agreement.

But Schakowsky is arguing that anti-deal groups will ignore members who’ve gone on the record in support and will focus their efforts on trying to sway undecided lawmakers. Still, besides the whip team, few members have come out strongly in support of the deal.

“My sense is that our members are really working to get to yes. I feel more optimistic every day that we’ll have enough votes to [sustain] the veto if that is necessary,” the Illinois Democrat said. “We’re encouraging members who have made a decision to put out a statement. We are encouraging members to decide early to in some ways spare themselves a very aggressive summer.”

Obama’s personal outreach may be helping as well. A group of freshmen members who met with the president during a 90-minute briefing last week in the Situation Room said he is more engaged with the grueling work of lobbying than they’ve ever seen him. It’s clear that the president, the lawmakers said, deeply understands the details of the accord and is making the argument that this is the best deal for the U.S.

The president also warned the freshman lawmakers he met with last week — none of whom have committed to supporting the deal — that the White House would be closely watching their votes and no Democrat would get a pass for voting against it.